Delay is a crucial parameter in each kind of communication service, from simple telephony to advanced multi-media communication. Conversational services are characterized by inter activity between the users at the ends of the communication links. Delay, or more precisely, round-trip delay has an important impact on the feeling of inter activity and thus the quality of the service. The parties interacting via the communication network should have a possibility to interact in the same manner as if they were present at the same place. The communication means should thereby be as invisible as possible. A high round-trip delay leads to long response times. Such long response times in general disturb the flow of the conversation and may be interpreted by the inter actors as a hesitation from the other party or may lead to annoying inactive waiting periods. Modern human beings are used to receive quite rapid answers and even relatively short delays may cause frustration. A high round-trip delay thus degrades the inter activity significantly. Moreover, insufficient cancellation of electrical or acoustical echoes may impact the communication quality if the round-trip delay is too high.
On the other hand, on each link of the communication delay is required from a transmission point of view. In addition to the basic delay for the transmission itself, coding delay is needed both for increased source coding efficiency and for more efficient strategies to combat transmission deficiencies. Generally speaking, coding delay which can be used for source coding, channel coding, interleaving, coded modulation, etc., gives a coding gain by which the transmission power can be reduced without degrading the transmission performance. In the context of the present disclosure, even delays for jitter buffering in case of packet transmission is considered as coding delay. Delays are therefore important and even necessary for providing problem-free transmission. All delays on one link sums up to the end-to-end delay, sometimes also referred to as mouth-to-ear delay. The round-trip delay is further the sum of the end-to-end delays of a original transmission and the reply thereon, respectively.
From the transmission point of view, delays are beneficial, but for providing an acceptable service quality for conversational services, delays have to be kept within certain limits. The coding gains achieved with the coding delay therefore have to be traded against the loss in inter activity due to resulting round-trip delay.
In some way all communication services trade delay against coding gain. AN example from speech communications with circuit-switched transmission is given with the ARM telephony service in today's GM system. The source coding involves a delay of 25 ms, the channel coding including interleaving on the FR speech traffic channel a delay of 37.5 ms. On top of that come certain delays, e.g. for digital and analogue signal processing, radio burst generation, modulation and demodulation, radio transmission, serial transmission on the Abis interface between base station and transcoder, etc. These delays are fixed during the speech connection.
An example from speech communications with packet-switched transmission is given with telephony services in the Internet. Here, the end-to-end delay from analogue signal input at the sending side to analogue output at the receiving side is, disregarding other contributors, caused by the transmission time of the data packets from sender to receiver, by jitter buffers which aim to cope with transmission delay variations, and sample synchronization techniques which align the asynchronous sample clocks at sender and receiver to each other. The jitter buffer delay at the receiver can be regarded as equivalent to the delay due channel coding in the example of circuit-switched transmission. In contrast to that case, the end-to-end delay can be varying. These variations are caused by adaptive jitter buffer techniques, which try to minimize the number of packet losses while keeping the end-to-end delay low. Changes of the transmission conditions e.g. due to congestion may lead to adjustments of the jitter buffer size, which in turn results in a variation of the end-to-end delay. There exist more or less advanced techniques for sample synchronization, which are also applicable when modifying the jitter buffer size. This ranges from simple insertion or deletion of signal samples to time-scale modification, which allows playing out the signal at different speeds without causing noticeable artifacts.
Half-duplex communication channels can carry data in either direction but not both directions at once. This implies that if a half-duplex channel shall be used for a speech communication, then only a signal originating from the active partner is transmitted to the passive partner but no signal is transmitted from the passive to the active partner. Such way, the transmission resource is shifted between the communication partners depending on their present activity. Communications via half-duplex channels make use of an asymmetry property according to which only one conversation link is active at a time. The inter activity on half-duplex channels is thereby severely limited, since a strict one-at-a time principle has to be fulfilled.
In U.S. Pat. No. 6,182,032 B1, a communication method applicable to half-duplex or one-way communication is presented. In one-way communication, the actual delay is of minor importance since no feed-back is present, and therefore a signal delay is allowed to increase, if the link is found to be a one-way communication link. It is thereby possible to modify e.g. encoding delays and to decrease the bit rate. This will reduce the demand for radio resources and result in a more efficient use of the available system resources. The method may be useful where one-way communication is to be expected, e.g. in cases of voice mail, message services, i.e. when a terminal acts as a source or sink for voice data, but not both. It is, however, concluded in the very disclosure that this solution is not applicable to full-duplex situations.